Generally, when multiple devices (bands) broadcast their wireless network with the same name (SSID), you can only see one network name in the wireless list of you end-devices.

For example, if the SSID of your router is 'TP-Link_Router' , and you also configure your Range extender with the same SSID, you will only see 1 wireless name in the list of your phone, though your router and extender have 2 wireless signals in total. And then, if you connect your phone to this network, you may not know which front device your phone is connecting to.

How to check?

After your extender has already been connected to the main router, and the signal/RE LED is lit:

If you haven’t manually change the SSID of the extender, the default SSID will be the same to the main router, of add the “_EXT” in the end of the router’s SSID. For example, if the SSID of your router is “TP-Link_Router”, the default SSID of the extender would be “TP-Link_Router” or “TP-Link_Router_EXT”. So, you can just check the wireless list of your phone, to check if you can find the specific SSID of your Range Extender.

If you have manually change the SSID of the extender, it is what you just configured.

How to configure the SSID of the extender again?

You can login the webpage of the extender to configure it again, or just reset it and follow quick setup wizard to set up it again.